The present invention relates to the gaseous detection arts. It finds particular application in conjunction with metal oxide sensors for detection of carbon monoxide, and will be described with particular reference thereto. It should be appreciated, however, that the invention is also applicable to the detection of other gaseous reducing chemicals, such as hydrogen, hydrogen sulfide, hydrocarbons, and organic vapors, including toluene.
The quality of indoor air affects the health and well-being of building occupants. Concerns over the control and improvement of indoor air quality have lead to the development of a number of gaseous sensors capable of detecting toxic and pollutant gases, such as carbon monoxide, carbon dioxide, hydrogen sulfide, chlorine, nitrogen oxides, ammonia, and sulfur dioxide, as well as combustible gases, such as hydrogen, methane, and other flammable organic vapors.
Metal oxide-based sensors using oxides of zinc, tin, titanium, and other semiconductive oxides have been evaluated for their abilities to detect specific gases. Tin oxide-based sensors have shown particular promise as they exhibit a high sensitivity to certain gases at relatively low operating temperatures. Such sensors detect gases by exhibiting a measurable change in the resistance of the bulk oxide when a gas or chemical vapor is adsorbed onto the surface of the oxide.
The sensitivity of a sensor is influenced by the microstructure of the sensing surface. Surface area to volume ratio, grain size, and pore size of the metal oxide particles which comprise the surface are understood to affect the performance of the sensor.
Thin film sensors are desirable because of their relatively small size and low power consumption. Such sensors may be prepared in a number of ways, including sputtering, physical vapor deposition, and chemical vapor deposition. Sputtering and physical vapor deposition techniques produce relatively thin films, of the order of a few hundred nanometers or less. Although such films exhibit good sensitivity to gases to be detected, they often have poor stability due to their low mechanical strength. Sensors produced by chemical vapor deposition tend to suffer from poor film uniformity as the film often shows an "island" texture.
Recently, sol-gel technology has been developed for the preparation of tin oxide powders. Two sol-gel synthesis routes are known. One route involves hydrolysis of tin alkoxide to the oxide and is discussed by Wilson, et al. ("Sol-Gel Materials for Gas Sensing Applications", Sensors and Actuators B., 18-19 pp. 506-510 (1994)) and Takahata ("Tin oxide Sensors, Development and Applications," in Chemical Sensor Technology, Vol 1, pp. 39-55 (Seiyama, Ed. 1988)). Another route employs hydrolysis of tin (IV) chloride, as disclosed by Vogel, et al. ("Quantum-sized PbS, CDs, Ag.sub.2 S, Sb.sub.2 S.sub.3, and Bi.sub.2 S.sub.3 Particles as Sensitizers for Various Nanoporous Wide-Bandgap Semiconductors", J. Phys. Chem., Vol. 98, pp. 3183-3188 (1994), and Mulvaney, et al. ("Electron Transfer in Aqueous Colloid SnO.sub.2 Solutions," Langmuir, Vol. 6, pp. 567-571 (1990)).
The present invention provides a new tin oxide sensor, having improved long-term stability and reproduceability, for rapid detection of carbon monoxide and other pollutant gases, which overcomes the above referenced problems and others.